It has long been known that shades of gray can be produced in ink jet recording by various techniques. U.S. Pat. No. 3,604,846 granted to D. Behane et al. on Sept. 14, 1970, teaches the technique in which a matrix of, for example, nine dots is formed with each dot being of the maximum density of the ink jet mechanism. In the Behane et al. patent a lighter gray shade is formed by recording or printing fewer dots within a matrix area. A darker gray shade is produced by recording more dots within the matrix area. All dots have the same size or density. U.S. Pat. No. 3,373,437, granted on Mar. 12, 1968, to R. G. Sweet teaches the formation of gray shades by depositing different numbers of drops at the various dot locations on the copy to vary the dot size and thus darkness or tonal density. Both of these systems experience some limitation.
In the system of the Sweet patent using a variable number of drops, a prohibitively large number of drops is required to form each dot in order to achieve the maximum degree of darkness while permitting sufficient gradations in between.
Likewise, the system of the Behane patent has distinct drawbacks because a matrix of equal-density dots selectively applied is also generally very coarse. If the matrix cell size or area is made large enough to contain a sufficient number of dots for an acceptably large range of gray tones, the reproduction lacks fineness of detail and is rather crude in appearance.
Furthermore, a visually linear progression of shades of gray (approximately equal density or a Munsell distribution) results from neither a linear progression of numbers of dots in a matrix cell or area nor from a linear progression of numbers of drops of ink to make up a dot.
Actually, the change from no drops of ink to one drop of ink or from no dots in a matrix cell to one dot is a much more pronounced change in the visual perception of the shade then the change from one less than the maximum number of dots or drops to the maximum number of dots or drops. That is, each unit of ink has a far greater visual effect at the light end of the tone scale than at the dark end. Therefore, in each of the systems, fewer visually equal tones are possible than the number of dots in a matrix cell or drops in a dot would at first suggest.
It is an object of the present invention to produce an acceptably large range of gray tones from a dot printing device such as in ink jet recorder with a minimum variation in the size or density of each printing dot or mark.
Another object of this invention is to maximize the resolving capability of the recorder, especially at the dark end of the tone scale.